Fix cgroup assignments for the threads

Check for sys.boot_completed and then assign
the cgroup for snapshot-merge and readworker threads.

Bug: 385659495
Test: OTA and verify cgroups are set for the threads
Change-Id: I7336d13e9e7a48c8917a1aa9c3bf2351b7524ed3
Signed-off-by: Akilesh Kailash <akailash@google.com>